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Solar Hybrid Hot Tub 

Make] Projects 

Solar Hybrid Hot Tub 

Written By: Eric Muhs 


Caulking gun (1) 
Drill (1) 
Paintbrush (1) 

Screwdriver (1) 
Utility knife m 


Plywood (2) 

Plexiqlass (2) 

Vinyl hose (3) 

Wood (Scrap) 

/ used some 2x2 lumber ends. 

Paint (1) 


/ used a 12V DC marine pump; a $20 

fountain pump that runs off AC would 

also work. The less water the pump 

moves, the hotter it heats, so no need 

for a monster. 

Solar panel (1) 

to run the pump; 1 aot mine from eBay. 

Hose barb (2) 

to connect the pump to the vinvl hose. 

Mav vary dependinq on your pump. 

Male-to-male hose barb (3) 

to join vinyl hose, plus extras for repairs 

Wood screws (24) 

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Solar Hybrid Hot Tub 

Wood screws (48) 
Carpenter's glued) 
Caulk (1) 


Back when I lived in California, I heated the water for my house and hot tub using some 
1970s "energy crunch"-era solar thermal collectors that I found at a junkyard. They were a 
nice design from Israel, with a large plexiglass collection surface and an insulated horizontal 
tank with its outlet up top, where the hottest water rises. 

Those restored units were all the water heating we needed for about 9 months of the year, 
and I installed them on a hillside rather than the roof for easier maintenance. (Alternative 
energy means lots of repairs; if you're harvesting energy from the environment, your 
equipment will be, well, out in the environment.) 

Then I moved to a newer house in Seattle with a fancy automatic hot tub on the rooftop deck. 
But I was appalled when I saw that it cost up to $40 in electricity each month to heat this 
mass of water and leave it covered outside overnight. 

So I came up with this simple and inexpensive solar system that adds heat to the hot tub 
during the day so that the main electric heater doesn't have to work as hard after the sun 
goes down. 

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Solar Hybrid Hot Tub 

Step 1 — Solar collectors. 

• The system heats the water by running it through coils of black vinyl hose inside 2 solar 
collector boxes. A small solar-powered pump draws water out of the tub, runs it through 
the coils, then dumps it back in. The tub's original heater and thermostat are not altered, 
but the heater switches on less. 

• For the solar collectors, I built 2 open 3'x3' boxes out of plywood reinforced with blocks of 
scrap wood in the corners (I had just 1 box originally, but expanded the system later). I put 
the boxes together using 1 1/4" wood screws and glue, then caulked the cracks and 
painted them matte black inside and out. For the vinyl hose inlet and outlet, I drilled a hole 
in the side of each box near the bottom. 

• I tucked the pump under one side of the hot tub, and used hose barb fittings to connect 
each end to black vinyl hose. Then I connected the rest of the hose together, coiled it 
inside both boxes, and dropped each end into the tub, separated slightly. The pump sits on 
the inlet side of the hose, but I don't think it matters.. 

• I fitted plexiglass tops onto the boxes with 3 A" screws, washers, and more caulk, and tilted 
the boxes up on the deck across from the hot tub, to lean south for more sunlight. 

• The vinyl hose is inexpensive and easy to repair in case of freezes: just cut out the broken 
part and insert a barbed connector. 

• TIP: Dipping the hose into very hot water for 15 seconds makes it easier to slip a 
fitting inside. 

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Solar Hybrid Hot Tub 

Step 2 — Power pump. 

• My original system had just 1 
collector and used an aquarium 
pump on a 1-hour timer. But the 
pump ran even on cloudy days, 
when it drew hot water out of the 
tub to get cooled — which was not 
the idea. 

• I thought of adding a light-sensitive 
switch to prevent this, but instead 
chose to power a new DC pump 
with a solar panel. The pump sits 
under the tub and only runs in 
bright sunlight, when the boxes 
heat the water most efficiently. 

Step 3 — Results. 

• This system doesn't provide the same level of heating as my old setup in California. Its 
surface area is smaller, and with less sun and colder temperatures, the Seattle weather 
doesn't help. As my old California neighbor Steve said, "Here, you can just throw hose on 
the ground." 

• But on sunny days, the days when your parked car is hotter than the outside air when you 
get in, it works great. The water returns to the tub 2° or 3° hotter than when it left, which 
may not sound like much, but it adds up. 

• The entire system cost me about $70 for parts and materials, and it will keep paying back 
as fuel prices rise. In warmer, sunnier climes, systems like this can actually overheat your 
tub. Unlikely in Seattle, but I did get my hot tub too hot once or twice in California. It cools 
off pretty quickly with the top off. As my physicist friend Heyward once said, "Heating 
water is not rocket science." 

This project first appeared in MAKE Volume 18 , page 135. 

© Make Projects 

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Solar Hybrid Hot Tub 

This document was last generated on 2012-11-03 05:31:05 AM. 

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